Rigify can't generate my rig
You probably deleted bones from the face. Rigify needs all of the facial bones to generate the rig.
Or you may have disconnected some of the bones that need to stay connected.
I have made a video about the face rig
https://www.youtube.com/watch?v=n8aCBKjPJRE
There is also a whole playlist of videos where I tried to explain how to avoid most of the annoying errors that Rigify throws at you.
https://www.youtube.com/playlist?list=PLdcL5aF8ZcJv68SSdwxip33M7snakl6Dx